I’ll preface this with the fact
that I have never experienced SAG; but it is an interesting subject. I don’t
run much 100LL, and I change the plugs at about 40 hours. A couple questions
come to mind:
Is all of the discussion of the copper
core, etc; relevant to a specific brand/part# plug? What heat range?
I use B9EGV racing plugs – do they
have a copper core? It’s a fine wire electrode; fairly high heat range
number (‘colder’ plug).
If the issue is preignition – is that
consistent with the large drop in EGT?
Al
-----Original Message-----
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Steven W. Boese
Sent: Thursday, September 30, 2010
7:14 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: New SAG
Theory Discussions of SAG
Just
a couple of clarifications: The decrease in the thermal conductivity of
the copper core may be the result of thermal cycling causing development
of porosity within the copper. This is independent of the corrosion
going on between the Mn alloy electrode sheath and the insulator. So
there are two essentially independent mechanisms contributing to the heat range
change of the spark plug with age.
The
development of the porosity within the copper is not particularly affected by
the type of fuel whereas the corrosion between the Mn alloy sheath and the
insulator is accelerated by the use of 100LL.
The
cure is still to install new spark plugs.
I
have colder spark plugs from Racing Beat that are recommended for prolonged
use in high RPM high power applications. At best, the colder plugs
will merely delay the onset of SAG but won't prevent it from happening
eventually. It will be some time before the results of using those plugs
will be known.
In a nut
shell, Steve's excellent data gathering (how many of use have access to a
scanning electron microscope or spectrum analyzer {:>)) and analysis
indicates a possible alternative reason for SAG other than fouling.
It
basically boils down to a theory of pre-ignition based on the deteorating heat
removal capacity of the plug.
There is
a copper core (the photos showing the two cores) in the center of the ceramic
cone which is responsible for conducting 90% of the heat away from the center
electrode. Based on Steve's' electron microscope viewing of two cores
(one new and one having been used in a 100LL environment), it appears that the
plug using leaded gasoline has corrosion occurring on the boundary between this
copper core and the surrounding (hot) elements. This layer of corrosion
hinders heat removal from the plug leading to parts of the plug becoming hot
enough to trigger pre-ignition.
This SAG
tends to happened at high power settings and reducing power tends to make it go
away (for a while). Steve's theory is that the plug encounters max heat
and temperature at high power settings and that reducing the power permits the
plug to cool off enough for the pre-ignition to go away (for a while).
So we
now have two theories - one is the fouled plug and one of the overheated plug
causing SAG. Preignition generally leads to detonation which does not
appear to be happening (at least no one is finding the typical detonation
damage to the rotary engine). A possible reason is the quenching effect of
the less than optimum shape of the rotary combustion space may tend to delay or
prevent the onset of detonation damage that one would expect from continuous
preignition caused by a too hot plug.
I
believe Steve intends to see if he can find some colder plugs (the ones that
conduct heat better) and see if that extends the time between SAG.
Currently I find that roughly 25-30 hours is about the max I can get using
100LL before SAG sets in, whereas Tracy and others have well over 150 hours
using unleaded Mogas. So if the colder plugs extend the time before SAG,
I would say that pretty much would suggest Steve's theory is correct - if Not,
then perhaps the fouled Plug theory still has merit or it could be some other
phenomena happening.
Steve,
my apologies if I did not present your case as well as you could
Sent:
Thursday, September 30, 2010 9:59 AM
Subject: [FlyRotary] Discussions of SAG
I understand that there
was a presentation on SAG at the meeting. Can someone go over the
alternative theory with the ones of us that were not in attendance? It
looked like someone had a scanning electron microscope photo that they were
showing! Wow! Are we getting sophisticated or what!! Was this
an explanation of the cause, or ideas for prevention, both??
Bill B